I agree with a few of the other "calmer heads" on this thread- Beating up on Ed "Da Torch" Lander and his personal experiences with double guns is like trying to teach a pig to sing German opera-- You get frustrated and it just annoys the Hell out of everybody- there are plenty of other venues for personal attacks, let's take the high road.
I agree with Herr Dudley here. I have two older 12 ejector doubles, one with double triggers, and one with a single selective trigger, and both have doubled a few times, but i found out the cause without any diss-assembly required.
Case 1-- 1945 pre-Marlin (Feb with hang tag) 12 LC Smith Ideal FWE 26" barrels, DT-- I bought this from a local dealer at the "Yooper" shoot, after my 12 GHE developed recocking problems (since cured by Brad Bachelder, my preferred gunsmith) and I needed a 12 double to complete the SC program. I was using RST 2.5" 1 ounce No 8 shot in the Parker, and continued to shoot that fine load in the Smith- A-OK. Later at home, using it for barn pigeons, I put a AA Handicap Trap load in the right barrel, and when I touched the trigger, there was a boom-pause-boom- so it had doubled. The gun is as new, hadn't been shot in years, and a removal of the lock plates showed no wear at sear bent or crud, so I went back to the RST lighter loads (always a safer bet with a Smith IMO) and no problemo.
I also have my Dad's 12 M21 field gun he got new in 1948-- SST, EJ--and with the trigger selector button set for right barrel first, no problem- if you move it to fire the left barrel first, about 1 in 10 times it will double, but only in that setting- As it is choked Mod right and Imp Mod left, I just leave it set for right barrel first-
Guns, like people, have their "quirks" and the "Gremlin" factor also can play a part. I just accomodate myself to those "quirks" and live with them--RWTF